When gathering, and moreso when gathering pieces longer than 3"-4" on a garment, I like to place scotch tape (the matte finish "magic tape by 3M) on top of the gathers, finger press the area flat, then sew a row of short stitches on top of the scotch tape, 1/16"into the ultimate construction SA. Then I peel off the scotch tape and continue. I feel this helps keeping the gathers prim and proper and prevents distortion.
It is important to use the matte finish tape since it is easy to peel off and you an use it on delicate fabrics as well.